1. The Field of the Invention
The present invention relates to glass-based joining materials, in particular amorphous or partially crystallizing joining materials with and without a filler material, which are generally also referred to as glass solders, and are suitable in particular for high-temperature applications, and to the uses thereof.
2. Description of the Related Art
Joining materials of this type are commonly used to produce joint connections, in particular to join ceramic components to one another or to join metal components in an electrically insulating manner. In the development of such joining materials, their compositions are often selected so that their respective coefficients of thermal expansion correspond approximately to those of the components to be joined therewith, in order to obtain permanently stable joint connections. Compared to other joint connections, for example those made of plastic, those based on glass have the advantage that they are configured to be hermetically tight and can withstand higher temperatures.
Such glass-based joining materials of the generic type are generally often produced from a glass powder which is melted in the soldering operation and, under the action of heat, gives rise to the joint connection with the components to be joined. The soldering temperature is generally selected so as to correspond approximately to what is termed the hemisphere temperature of the glass, or can differ therefrom usually by ±20 K. The hemisphere temperature can be determined in a microscopy process with a hot-stage microscope. It denotes the temperature at which an originally cylindrical sample has amalgamated to form a hemispherical mass. The hemisphere temperature can be assigned a viscosity of approximately log η=4.6, as can be gathered from appropriate specialist literature. If a crystallization-free joining material in the form of a glass powder is melted and cooled again so that it solidifies, it can usually also be melted again at the same melting temperature. For a joint connection with a crystallization-free glass solder as the joining material, this means that the operating temperature to which the joint connection can be subjected in the long term must not be higher than the soldering temperature. In actual fact, the operating temperature in many applications has to be significantly below the soldering temperature, since the viscosity of the glass solder decreases with increasing temperatures and a glass having a certain degree of flowability can be pressed out of the joint connection at high temperatures and/or pressures, and therefore the joint connection can fail in its function. For this reason, glass-based joining materials suitable for high-temperature applications usually must to have a soldering temperature or hemisphere temperature is which is significantly above the later operating temperature, and/or they must crystallize sufficiently, such that the crystallization ensures an adequate stability of the solder during operation at elevated temperatures.
One field of use of such joining materials is, for example, that of joint connections in high-temperature fuel cells, which can be used, for example, as a power source in motor vehicles or for localized power supply. An important type of fuel cell is, for example, what is termed an SOFC (Solid Oxide Fuel Cell), which can have very high operating temperatures of up to approximately 1100° C. The joint connection is in this case commonly used for producing fuel cell stacks, i.e. for joining a plurality of individual fuel cells to form a stack. Fuel cells of this type are already known and are continuously being improved. In particular, the trend in present-day fuel cell development is generally towards lower operating temperatures. Some fuel cells already achieve operating temperatures of below 800° C., and therefore a reduction in the soldering temperatures is possible and also desirable on account of the resultant low thermal loading of the SOFC components during the soldering process.
A suitable metal for producing SOFCs, in particular interconnectors, are, for example, chromium-containing alloys, such as CFY (chromium-iron-yttrium, Cr5FeY), which is sold by Plansee as a chromium-based alloy having an iron proportion of 5% and small quantities of yttrium oxide. Glass-based joining materials used, for example, in SOFCs must be compatible with the materials used there for the joining partners, i.e. for example with CFY.
DE 10 2009 038 812 A1 describes crystallizing glass solders which can withstand high temperatures and which contain up to 40 mol % BaO and only very small amounts, of 0-2 mol % CaO. These materials are difficult to process; it has not been possible to produce reliable joint connections, particularly for SOFC applications, using these.
Alkaline earth metal aluminosilicate glasses are described in patent specification U.S. Pat. No. 6,532,769 B1. However, they have an unfavourably high content of BaO and Al2O3, which leads to the formation of polymorphic celsian phases.
DE 10 2011 080 352 A1 describes soldering glasses for use at high temperatures, which are based on alkaline earth metal aluminosilicates and have a high CaO content. On account of their very low SiO2 content, their flow properties before crystallization are greatly reduced.
U.S. Pat. No. 5,998,037 A describes porcelain enamel based on BaO, B2O3 and SiO2 for electronic applications. The glasses described do not contain any CaO, however, and preferably have a small SiO2 content and a high B2O3 content. In addition, they can also contain further oxides of alkali metals, fluorine, phosphorus oxide or else polyvalent oxides such as Co3O4, which are all disadvantageous for the resistance of the glass, e.g. under stack-related conditions of an SOFC (high temperatures and also humid and reducing gases).
U.S. Pat. No. 6,124,224 A describes glass-ceramic joining material for use in the high-temperature range in particular for producing hermetic joints for sensors in the exhaust-gas field. The glasses described in this document have relatively high Al2O3 contents, which, on account of the possible formation of monocelsians and cordierite, are disadvantageous in the case of high operating temperatures and operating times.
DE 10 2009 011 182 A1 relates to soldering glasses with crystallization properties, the total BaO and alkaline earth metal content of which is very high. High contents of barium in combination with chromium in the high-grade steels or chromium alloys commonly used lead to the strong formation of an anisotropic barium chromate phase mismatched in terms of expansion at the interface, which is observed as a reduced cycle resistance.